It was surprising, yet no one was surprised. It was stunning, yet at the end, Jeff Green was hardly stunned, more disappointed that his heroic effort didn’t lead to a Celtics victory.

Green scored 43 points in Boston’s 105-103 loss to the rival Miami Heat Monday night at the Garden, a career high for points and perhaps his career-defining game, a night that scores of Celtics fans believed would never occur.

Their confidence in Green had been wavering, especially after his uneven first stint with the Celtics in 2011 and then a topsy-turvy beginning to this season, after missing an entire season following surgery to repair an aortic aneurysm.

The only residue from that surgery is a 9-inch scar down the middle of his chest. But there remained sour thoughts from his previous stint in Boston. The club had invested $36 million and carved out a special role to allow Green to get comfortable.

Green’s maturation as a player has been slow, and Doc Rivers has been patient but persistent with his young forward. On Monday night in front of a national television audience watching to see if the Celtics could even threaten the Heat’s 22-game winning streak, Green scored 26 points in the first half on an array of one-handed looping runners, long 3-pointers, and midrange jump shots.

“He was locked in,” Miami guard Dwyane Wade said. “I think he’s a very good player. I think tonight he was used — well, he’s used to being used — but he was on fire. He just came out very aggressive and they had to have somebody score a lot of points. He had his opportunities.”

He was unstoppable, carrying the undermanned Celtics to a 17-point first half lead, one they would eventually lose down the stretch. And the only thing that stopped him in the second half was 4:14 of rest in the fourth quarter. It was a rest that proved costly.

The Celtics were outscored, 10-3, in that stretch and Green did not score when he re-entered the game. With Kevin Garnett out and Paul Pierce playing mostly distributor and defender, Green was the team’s primary scoring threat. For one night, he was the go-to guy, a responsibility he has been resistent to accept as Garnett and Pierce age.

Such scoring binges are why the Celtics decided to reward him like a franchise cornerstone. In the past month, he has begun to unleash his potential.

He entered Monday averaging nearly 15 points since the All-Star break, impressing his teammates — and opponents — with his growing confidence.

Green is no secret around the league. He is 6 feet 9 inches and 235 pounds of pure athlete with a long wingspan. He’s too fast for power forwards and too big for small forwards, which is why the Sonics selected him with the fifth pick in the 2007 NBA Draft they acquired from the Celtics.

What has stopped Green from becoming a star is his passive personality. He freely relented to Kevin Durant and Russell Westbrook in Oklahoma City and has done the same here to Pierce, Garnett, and Rajon Rondo.

Rivers has pushed him to seize the moment, fulfill his potential, and develop a mean streak.

“You’ve got to find different ways to attack. You’ve got to find different ways to help your team out,” Green said. “So I mean the next game is going to be a lot different than this game, so I just have got to figure out another way, of how I can continue to stay aggressive.

“I’m mad we lost. It sucks, but we’ve got to move on.”

Green would only talk team. He wasn’t interested in crowing about his landmark night. The Celtics lost and he didn’t score for the final 9:06. When the game needed his imprint in the final seconds, he came up short, thanks to a brilliant defensive play by Shane Battier, who stripped Green of a potential layup attempt with seven seconds left and the Celtics trailing, 105-103.

That would be his last chance to score. LeBron James, who scored 13 of his 37 points in the second half, finished the job with a jumper with 10.5 seconds left. Green learned the hard way that sometimes 43 isn’t enough. He and the Celtics needed another play, another big basket.

Green measured tall amongst the trees but he wasn’t the tallest. The tallest was James, who took over when his team trailed by 13 with 8:27 left. By then Green was exhausted, asking for a break in the midst of the Celtics upset bid.

James played the final 9:06, slicing the Celtics with big baskets while Green got his rest but couldn’t carry his team when he returned. Monday was a triumphant night for Green, but not triumphant enough. He needed to finish and he was the only Celtic capable of such feats, so his postgame disappointment was understandable but the optimism about his potential is brimming.